Separators of the type that arrange a plurality of multiple tube-type separators in a parallel flow arrangement for removal of particular material from fluid streams are well-known. U.S. Pat. No. 2,941,621 and 2,986,278 and 3,061,994 generally disclose such separators and the individual tubular centrifugal separators contained therein. These arrangements house the inlets to the separators between upper and lower tube sheets that also retain upper and lower portions respectively of the tubular separation elements. Specific design criteria for such arrangements are well known. Other variations of the individual centrifugal separators and the separator layout itself can be found in U.S. Pat. No. 3,443,368 and 5,690,709.
These separators are commonly used to separate fine, particulate material from high temperature streams associated with the use of fine, particulate catalyst. One such process is the fluidized catalytic cracking process. The fluidized catalytic cracking of hydrocarbons is the principal process for the production of gasoline and light hydrocarbon products from heavy hydrocarbon charge stocks such as vacuum gas oils or residual feeds. The FCC process is carried out by contacting the hydrocarbon feed with a catalyst made up of a finely divided or particulate solid material. In the FCC process large hydrocarbon molecules associated with the heavy hydrocarbon feed are cracked to break the large hydrocarbon chains thereby producing lighter hydrocarbons. These lighter hydrocarbons are recovered as product and can be used directly or further processed to raise the octane barrel yield relative to the heavy hydrocarbon feed. The basic components of the FCC process include a reactor, a regenerator and a catalyst stripper. The reactor includes a contact zone where the hydrocarbon feed is contacted with a particulate catalyst and a separation zone where product vapors from the cracking reaction are separated from the catalyst. The catalyst is transported like a fluid by passing gas or vapor through it at sufficient velocity to produce a desired regime of fluid transport. During the cracking reaction, coke will be deposited on the catalyst. Coke interferes with the catalytic activity of the catalyst by blocking active sites on the catalyst surface where the cracking reactions take place. Catalyst is traditionally passed to a regenerator for purposes of removing the coke by oxidation with an oxygen-containing gas. Oxidizing the coke from the catalyst surface releases a large amount of heat, a portion of which escapes the regenerator with gaseous products of coke oxidation generally referred to as flue gas.
The flue gas generated by combustion of coke from the FCC catalyst produces a high temperature gas stream that, after passage through cyclone, still retains a substantial amount of fine catalyst particles that must be removed prior to further treatment or energy recovery from the flue gas. The multiple tubular separator arrangements are particularly suited to the removal of this very fine particulate material.
Application of multiple tubular separators for removal of dust and particulate material from such high temperature fluid streams has required further consideration and special arrangements. FIGS. 1 through 3 of U.S. Pat. No. 4,863,500 and of U.S. Pat. No. 3,541,766 each show a separator arrangement that is suitable for the usual high temperature operations of processes such as the FCC process. The three different arrangements of U.S. Pat. No. 3,541,768 and 3,415,042 each suspend multiple centrifugal separators from an inner housing to accommodate different expansion rates between the internals of the separator and an outer housing. Internal insulation normally covers the inside of the outer housing to reduce its wall temperature thereby increasing its strength for pressure containment, but reducing its expansion relative to the internals.
FIGS. 1 and 3 of both of the above patents show a completely closed inner housing contained within an outer housing vessel. In these two arrangements the inner housing retains the separation elements in a closely conformed housing that introduces a number of design complexities to the overall arrangement. Complexities associated with these arrangements involve the use of expansion elements, additional inlets or outlets, and purge requirements for retaining all of the separate centrifugal separators in a completely enclosed housing. In some cases the complexity of the separators can impose additional pressure drop from the total system.
FIG. 2 of the '766 patent and of the '042 patent shows a less confined separator arrangement. The generally simplified arrangement of the separator shown in FIG. 2 has lead to its adoption in different forms in most commercial applications.
To provide flexibility for the radial growth of the tube sheets supporting the tubular separators, the more open separator arrangement results in a significant space between the wall of the containment vessel and the closest outlet of the tubular separators. Up until this time, the containment of the gas stream below the outlets of the tubular separators in this type of arrangement has received little attention. This is particularly true in the arrangement of the more open-type separator as shown in the above mentioned FIG. 3.
It is a constant goal in the use of the multiple tube separators to improve separation efficiency. Separators will usually operate with a separation efficiency of over 60%--with some providers of such separators claiming efficiencies up to 90%. The separation efficiency is defined as the difference between the particles entering the separator and the particles leaving through the fluid outlet of the separator divided by the rate of particles entering the separator. The efficiency related to selected particle size ranges and the overall efficiency are both important for evaluating the operation of a separator with the efficiency in particular particle size ranges being particularly important in many applications.